1. Field of the Invention
This invention relates to the field of magnetic recording media, and in particular to multilayered magnetic recording media having large Kerr rotations in the blue wavelength region and improved perpendicular anisotropy and magnetic coercivities relative to those of conventional multilayer media.
2. Related Art
Magnetic recording media provide a convenient means for storing and retrieving data through the magnetization of local regions of the media in orientations to represent the ones and zeroes of binary codes. Typically, the magnetic medium is formed into a thin film and localized domains are magnetized in write operations by magnetic fields or thermomagnetic means. These localized domains can be read subsequently by measuring the polarization rotation of light scattered from the magnetic medium (Kerr rotation) or by measuring the magnetic field directly where the magnetic moment of the medium is sufficiently strong.
Much effort has been directed to enhancing the density of data storage in these magnetic media, as well as stability of stored data and the ease with which stored data can be read. For example, it is desirable to develop magnetic media having large magnetic coercivities, H.sub.c, since the magnetic moments of such materials require large magnetic fields for reorientation, i.e. switching between ones and zeroes. Thus, exposure of the magnetic medium to stray magnetic fields such as those generated during magnetic write operations is less likely to corrupt data stored at adjacent locations.
The density with which data can be stored on a magnetic thin film medium for perpendicular recording is related to the perpendicular anisotropy (K.perp.) of the material, which reflects the tendency for the magnetic moments to align in the out-of-plane direction. Thin film magnetic media having high perpendicular anisotropies have their magnetic moments aligned preferentially perpendicular to the plane of the thin film. This reduces the transition length between areas with moments of opposite orientation, allowing a larger number of magnetic bits (domains) to be packed into a unit area of the film and increasing the aerial density with which data can be stored.
A large perpendicular anisotropy is also reflected in a larger H.sub.c since the preferential out-of-plane alignment of the magnetic moments raises the energy barrier for the nucleation of a reverse magnetization domain and, similarly, makes it harder to reverse magnetic domains by rotation. Further, the magnetic remanence of a medium, which measures the tendency of the magnetic moments of the medium to remain aligned once the magnetic field is shut off following saturation, also increases with increasing K.perp..
The magnetic media used presently in magneto-optic thin film recording technology are usually amorphous thin film combinations of transition metals and rare earth metals (TM/RE) which have suitable Kerr rotations at red wavelengths. However, use of light having wavelengths in the red wavelength region to read TM/RF media limits the aerial density of data since the longer wavelength of red light has lower spatial resolution than, for example, blue light. Efforts are presently underway to develop Co/Pt or Co/Pd multilayer materials for perpendicular magnetic recording media since these materials have suitable Kerr rotations in the blue wavelength region. Of these materials, multilayers based on Pt/Co bilayers provide the better results since multilayers based on Co/Pd bilayers have smaller Kerr rotations than those based on Pt/Co bilayers. For example, Co/Pt multilayer films display H.sub.c on the order of 0.8 to 3 kOe, magnetic remanences of almost 100%, Kerr rotations at 400 nm of 0.25.degree. and K.perp.s of up to 6.times.10.sup.6 erg/cc(Co) (6 Merg per cc of Co). The Kerr rotations and K.perp.s of Co/Pd based materials are 0.18.degree. and 10 Merg/cc (Co) for comparable films. However, these properties are a strong function of the period and thicknesses of the Co/Pt and Co/Pd layers, and the sharpness of the transition between these layers. Furthermore, the best values of these parameters are obtained only with Xe or Kr sputtering, electron beam evaporation, or molecular beam epitaxy (MBE), each of which has significant drawbacks as a large scale manufacturing method. For example, Xe and Kr are more expensive sputtering gases than Ar, and Xe is known to poison cryogenic vacuum pumps. MBE requires expensive, complex equipment and has low yields. Evaporation techniques are slow.